It is well known that particulates of metals or metal oxides with sizes at nanometer level or submicron level are very useful industrial products in many fields of application. These applications include the manufacture of catalysts used in chemical industry, pottery and porcelain, electronic elements, coating, capacitor, mechanical-chemical polishing slurry, magnetic tape and fillers for plastics, paint or cosmetics.
It is possible to produce ultra fine particulates of metals or metal oxides by various technologies including high temperature gas phase method, mechanical method, chemical method and etc. Reviews on the general technology of the production of nano-particle were published in the following papers: V. Hlavacek and J. A. Puszynski, “Advances in the Chemical Industry of Advanced Ceramics”, Industrial Engineering and Chemistry Research, 1996, vol. 35, 349–377; “Advances on the Method of Preparation for Nano-particles”, Chemistry Bulletin (in Chinese), 1996, No. 3, 1–4. In CN 1217387A, there was also a detailed discussion on the advantages and disadvantages of the various technologies.
The process of the liquid phase precipitation method is simple. When compared with the gas phase method, solid phase method or other liquid phase method, its controlling condition is not so critical and its cost is lower. Therefore nowadays the liquid phase precipitation method is widely used.
The characteristics of the process of the conventional liquid phase precipitation method are as follows: stirring pot is used to carry out mixing reaction, and at least one of the reactant solutions is gradually added into the stirring pot by dropping, flowing in or atomizing for a relatively long time. Although this technology for preparing nano-particles has the advantage of simple operation, low cost and high yield, it has three generally recognized disadvantages as follows: (1) it is difficult to control particle diameter; (2) it is difficult to obtain very small particle diameter; (3) it is difficult to eliminate hard agglomeration among particulates. The origin of the drawbacks of the pot technology comes from the too long feeding time for one of the reactant solution and from the mixing of the reaction, product and precipitate formed at different stages of time while stirring. Nuclei formed at the initial stage will undergo growth and collision coalescence among small particulates to form nano-particles. Due to long time, nano-particles will grow to be relatively larger in size and will agglomerate together among nano-particles. The participation of the product formed in the later stages will induce agglomeration hardening. As mentioned above, these are the causes of the above-mentioned three drawbacks of the large pot technology in preparing nano-powder.
Therefore, various processes of liquid phase precipitation method for producing nano-powder without the use of stirring pot have been successively developed. Patent Application No. SE 99/01881 disclosed the following method and facilities: on the basis of a stream of carrier fluid flowing continuously in a pipe, two kinds of reactant solutions were injected in the form of periodical, intermittent pulse into the pipe at the same location. The reaction zone where the mixing of the injected two reactant solutions took place was separated in the carrier fluid. The lasting time for the course of mixing, reacting, and forming precipitate was very short. The invention claimed that the quality of the nano-particles was very good, with particulate size at 10–20 nm, slight inter-particulate agglomeration or even no agglomeration. The drawbacks of that method are: (1) reactant solutions are injected in a pulse mode and the mixing process is not continuous, thus the process is not favorable for large-scale continuous industrial production, and since carrier fluid must be used, the manufacturing process gets complex, it not only consumes carrier fluid but also needs a separation treatment for the carrier fluid and etc. and thus increases the production cost; (2) the method does not take any effective measures to reinforce and to adjust the mechanical mixing efficiencies of the two reactant solutions, therefore, it is impossible to effectively control the mechanical mixing efficiency of the reactant solutions. The above two drawbacks both shall be improved.
Other 2 papers, “Preparation of Strontium Carbonate Nano-powder by Liquid-Liquid Method in Rotating Packed Bed”, Science and Technology in Chemical Industry (in Chinese), 1999, 7(4) 11–14 and “Experimental Study on Microscopic Mixing in Rotating Packed Bed”, Chemical Reaction Engineering and Technology (in Chinese), 1999, 9, Vol.15, No. 3, 328–332, described another kind of continuous process without the use of stirring pot. Two reactant solutions were allowed to pass continuously through rotating packed bed at one time. In the rotating packed bed, two reactant solutions mixed, reacted, formed nuclei and formed nano-particles. The paper stated that under the action of super gravity, the reactant solutions passed through the rotating packed bed and were dispersed, broken by the packing and formed very large and continuously refreshing surface area, greatly reinforced the material transfer condition. Besides, the process of rotating packed bed has the advantage of high intensity of fluid passage and short resident time. However, there were still some drawbacks in the method of super gravity rotating packed bed. Due to the high compactness of the fillers such as steel wire net and in the packed bed, what obtained by the solution was not the action of stirring and shear. When solution entered into the packed bed, it as a whole rotated with the packed bed and obtained centrifugal force. Under the action of centrifugal force, the solution would flow from inner fringe of the rotor to outer fringe along the interstitials of the packing and in the course of this process, mixing of solution took place. The mechanical mixing intensity and the adjusting sensitivity of such kind of mixing were not high enough and thus the performance of the preparation of nano-powder was not ideal. Except nano-powder of CaCO3 and SrCO3, no report on the successful preparation of important species such as Zro2, TiO2 by using rotating packed bed was disclosed. Therefore the method seems to need further improvements.
As mentioned above, a good mixing and reacting facility for continuous passage of two reactant solutions should have the characteristics of high mechanical mixing intensity, adjustable mechanical mixing intensity and simplicity of structure. Within such facility, the solution should acquire vigorous stirring, shearing and turbulence and would quickly be separated and broken into isolated very small sized micro liquid agglomerates in order to enlarge the interface of the two solutions so as to provide good conditions for the processes of molecular diffusion, chemical reaction, nucleation and etc.
Chinese Patent Application No.01106279.7 filed on Mar. 7, 2001, which has not been disclosed and is entirely incorporated herein as a reference, provides a process without using a stirring pot comprising: continuously feeding reactant solutions into a dynamic rapidly and orderly micro liquid agglomerate mixing and reacting precipitator to separate the reactant solutions into a large amount of micro liquid agglomerates capable of rapidly mixing and reacting with each other in a turbulence state in reaction zones orderly arranged along the fluid direction to from a precipitate-containing slurry; feeding the precipitate-containing slurry from the mixing and reacting precipitator to a procedure of rinsing and filtering; and treating the resulting precipitates by various processes such as drying, heat treatment, pulverization and the like. The nano-particle is very fine and uniform. The hard agglomeration among the nano-particles is prevented.
Therefore, the objective of the present invention is to further improve the process disclosed in Chinese Patent Application No. 01106279.7 and to provide a further method of preparing nano-powder by liquid phase precipitation. The method of the present invention adopts a mixing facility which is simple in structure, can provide high and adjustable mechanical mixing intensity and can be used for large-scale production of good quality nano-powder. The method is widely applicable in the production of nano-powders of oxides, hydroxides, salts, metals and the like. Moreover, the number of devices can be decreased and the parameters to be controlled can be simplified.
After reading the following descriptions, this and further objectives, advantages and features of the present invention will become clearer.